A communications device generally comprises a transmission chain for a transmit signal and a receiver chain for a receive signal connected to an antenna and to a baseband processing device.
FIG. 1 shows a communications device 1 according to the prior art comprising a transmission chain Tx and a receiver chain Rx connected to an antenna ANT and a baseband processing unit 2, notably a baseband processor.
The transmission chain Tx notably comprises a power amplifier (or PA) device 3 one output of which is connected to the antenna ANT via the source-drain path of a transistor T1. The common node between the amplifier device 3 and the transistor T1 is connected to a ground GND via the source-drain path of a transistor T2. The input of the amplifier device 3 is connected to other conventional and known elements of the transmission chain (not shown here for the sake of simplification of the figure).
The receiver chain Rx comprises an amplifier device 4 commonly denoted by those skilled in the art as a low-noise amplifier (or LNA) one input of which is connected to the antenna ANT via the source-drain path of a transistor T3. The common node between the transistor T3 and the amplifier device 4 is connected to the ground GND via the source-drain path of a transistor T4. The output of the amplifier device 4 is connected to other conventional and known elements of the receiver chain (not shown here for the sake of simplification of the figure).
The amplifier device conventionally comprises at least one amplification transistor T.
The transistors T1, T2, T3 and T4 may be of the same type, for example MOSFET transistors with thick gate oxide, and may be of different dimensions. They are of any given nature, for example NMOS field-effect transistors. The switching circuitry provided by transistors T1, T2, T3 and T4 is controlled by a control circuit incorporated into the baseband processing unit 2.
In a signal transmission mode, the transistors T1 and T4 are conducting, and they behave as parasitic resistances Ron. The transistor T4 provides the grounding of the input of the amplifier device 4. The transistors T2 and T3 are open; they behave as stray capacitances Coff.
In reception mode, the transistors T1 and T4 are open and the transistors T2 and T3 are conducting. The transistor T2 provides the grounding of the output of the amplifier device 3. The transistors T1 and T4 behave as stray capacitances Coff. The transistors T2 and T3 behave as parasitic resistances Ron. The transistor T3 degrades the “Noise Figure” of the amplifier 4.
The quality of the switching device comprising the transistors T1, T2, T3 and T4 is described by the coefficient Coff*Ron. However, this coefficient is fixed for a given technology.
In transmission mode, the amplifier device 3 may, for example, transmit a signal with a power of 33 dBm, i.e. 2 W under a voltage of 10V. The stray capacitance of the transistor T3 allows a stray current to flow which supplies the amplifier device 4. The current coming from the stray capacitance of the transistor T3 can deteriorate the amplification transistor T. Consequently, the switching transistors are dimensioned in such a manner that they channel the parasitic power under a voltage of 10V; they therefore have large dimensions.
Generally speaking, each switching transistor T1, T2, T3 and T4 comprises a set of transistors assembled in series and in parallel in the form of a matrix. This disposition allows a configuration of transistors to be defined, for a given coefficient Ron*Coff, that limits the parasitic effects. The more transistors assembled in the form of a matrix the switching transistor T3 comprises, the more the transistor T3 protects the amplification transistor T.
However, it is desirable to find a compromise between the size of the matrix and a sufficient protection of the amplification transistor T. Indeed, the addition of transistors increases the number of stray capacitances Coff. Moreover, the parasitic resistance Ron of the transistor T3 degrades the noise figure of the low-noise amplifier device in reception mode.
The various elements of the switching circuit are incorporated into the same substrate. They are close to one another. Consequently, substrate and electromagnetic coupling phenomena appear. These phenomena may also degrade the noise figure of the low-noise amplifier device.
There exists a need to improve the noise figure of the low-noise amplifier device and to reduce the space requirement of the switching circuit while at the same time preserving this amplifier device from any deterioration.